Conformation and surface expression of free HLA-CW1 heavy chains in the absence of β2-microglobulin

A β2-microglobulin (β2m)-deficient kidney carcinoma cell line and three monoclonal antibodies to the α1 (L31), α2 (W6/32), and α3 (Q1/28) domain of class I HLA molecules were selected to assess the role of β2m in regulating the conformation and surface expression of HLA-C molecules. HLA-A2, -B27, and -CW1 molecules synthesized by β2m-deficient cells were compared to heavy chains synthesized in transfectants expressing a large excess of β2m. As assessed by differential binding with monoclonal antibodies and partitioning studies in the detergent TX-114, no HLA-A2, -B27, or -CW1 molecules can be expressed, in a correct conformation, by β2m-deficient cells. These cells, however, do express low but significant amounts of free HLA-CW1 heavy chains at the cell surface. Transfection with β2m causes a coordinate change in the antibody reactivity of the three domains of HLA-CW1 molecules, thereby providing the first experimental demonstration that assembly with β2m affects the folding of not only the al and α2, but also of the α3 domain. HLA-CW1 heavy chains, when free of β2m, are less soluble in the detergent TX-114 than free HLA-B27 heavy chains, and when associated with β2m share an α3 domain epitope with free HLA-A2 and -B27 heavy chains. Moreover, their assembly with β2m is largely incomplete. These data additionally demonstrate an impaired ability of HLA-CW1 to properly fold and establish a close similarity of HLA-CW1 to murine Db and Ld molecules. Although the functional role, if any, of free HLA-CW1 heavy chains remains to be determined, the present study demonstrates that the absence of β2m does not completely ablate class I expression in neoplastic cells of human origin.